Since commit e948509 ("setup.py - don't distribute tests"), tests files
are not included in the `.tar.gz` bundle on a fresh repo clone. (On old
repos they were still included, because listed in
`yamllint.egg-info/SOURCES.txt`.)
Let's explicitly include them.
Put `tests/yaml-1.2-spec-examples/*` back in `setup.py`'s `package_data`
because they need to be installed when running `python setup.py build`,
so Debian packaging script `dh_auto_test -O--buildsystem=pybuild`
doesn't fail.
See also commit e6dc67f.
`yamllint/conf/*.yml` remains in `setup.py`'s `package_data` because it
needs to be installed when running `pip install .`.
`docs/*` and `tests/yaml-1.2-spec-examples/*` just need to be packaged,
they can go in the manifest.
